Penny Blood: Serial Sensation and Working-Class Entertainment — page 298: what you’re looking at

Machine-transcribed from the original scan — historical spelling and the odd misread are preserved.

CH, xxIL} * SUMMARY. 281 temperatures; and the rate ought, as a rule, to be greater in lowlying than in elevated regions. This is known to be the case. Careful observations of temperature were made during the construction of the tunnel through Mont S. Gothard, and sufficiently reliable ones also at Mont Cenis. In both these tunnels the rate was found to be about 1° F. for 100 feet, which is only about half the usual rate. The rate being known, and the height of the mountain, and also the rate at a place elsewhere near the sea level, if we assume the relative densities of the crust and substratum to be those of granite and basalt, we can calculate the thickness of the crust and the melting temperature from these data, without making any assumption about the melting temperature. In this way we have obtained for the thickness of the crust about 25 miles at the sea level; and for the melting temperature about 2500°F. The first of these values agrees with that already estimated in X., and the latter is by no means im- probable from what we know about the melting temperatures of silicates. The calculations which we have made respecting the thickness of the earth’s crust at the sea level, necessitate a corresponding thickness beneath the ocean.’ The result at which we have arrived is that, if the crust were of the same density there as beneath the continents, it would have to be thinner than is possible beneath the oceans. We therefore con- clude that it is not of the same density beneath the oceans, and thus we are led, by another line of argument, to the same con- clusion as that mentioned in VII., viz., that the crust beneath the oceans is denser, and that it is to this cause that the ac- cumulation of water over one-half the sphere is to be attributed. If we put the mean depth of the ocean at three and a half miles, with our assumed values of the densities of the crust and substratum, we have found that on these data the thickness of the crust at the sea level cannot be less than about 25 miles; which so far agrees with the estimates before made. And in this last calculation no considerations of temperature whatever are involved, as they were in the former, and have always been in previous estimates. We conclude on the whole that the density of the sub- COLA @ NOO (CO)